[subset] further changes to serializer error handling.

- Rename enum type and enum members.
- in_errors() now returns true for any error having been set. hb-subset now looks for offset overflow only errors to divert to repacker.
- Added INT_OVERFLOW and ARRAY_OVERFLOW enum values.
diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index 132d008..062eea6 100644
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -102,7 +102,7 @@
     TRACE_SERIALIZE (this);
     if (unlikely (!c->extend_min (*this))) return_trace (false);
     if (unlikely (!coverage.serialize (c, this).serialize (c, glyphs))) return_trace (false);
-    c->check_assign (deltaGlyphID, delta, HB_SERIALIZE_ERR_OTHER);
+    c->check_assign (deltaGlyphID, delta, HB_SERIALIZE_ERROR_INT_OVERFLOW);
     return_trace (true);
   }